The present invention relates to a semiconductor device, a liquid crystal display device (hereinafter referred to as a liquid crystal display device) employing the semiconductor device and methods of fabricating the semiconductor device and the liquid crystal display device, and in particular to techniques of fabricating a thin film transistor (hereinafter referred to as a TFT) comprising a polycrystalline semiconductor on an insulating substrate.
There is a technique for fabricating peripheral circuits such as a driver circuit for driving pixels and a control circuit for controlling the driver circuit at the periphery of an insulating substrate on which pixels are fabricated in a liquid crystal display panel, for example.
The process for fabricating a polycrystalline Si TFT (hereinafter referred to as a p-Si TFT) of the peripheral circuits is intrinsically a hot-temperature process, but a low-temperature process for it is realized by using a process explained below.
The low-temperature process comprises formation of an amorphous silicon (hereinafter referred to as an a-Si) film, conversion of the a-Si film into a polycrystalline film by irradiation of excimer laser, formation of a gate insulating oxide film by plasma CVD or the like, formation of a gate electrode made of a metal or a metallic silicide by a sputtering method or the like, formation of source and drain regions by ion doping or ion implantation, and then ion activation by laser annealing.
The above crystallization of an a-Si film by excimer laser uses a phenomenon that irradiation of a UV light pulse of about 20 ns melts the a-Si film and then crystallization occurs as the a-Si film cools.
But with the conventional method, it is very difficult to control the grain sizes, orientations and positions of crystals in the polycrystalline film because of fast crystallization and non-equilibrium process.
The larger the grain sizes are, the better the performance of the p-Si TFT becomes, but the wider the spread in the grain sizes becomes and consequently the wider the variability of TFT characteristics becomes.
If the grain sizes are selected to be sufficiently smaller than the length of a channel of TFTs, the variability of the TFT characteristics becomes smaller, but the TFT characteristics are degraded.
The p-Si TFTs of the peripheral circuits in the liquid crystal panel are of the so-called SOI (Silicon-On-Insulator) type using an insulating substrate such as a glass substrate and are not capable of establishing a substrate potential, and consequently an adverse effect such as a projection called a "kink" occurs in a current-voltage characteristic curve especially of the p-Si TFT constituting the high-performance peripheral circuits.